Why do we only ever see one side of the moon on Earth?

People have known for a long time that the surface of the moon always looks the same, even if the shape of the moon changes over the course of a month. They therefore said that the moon had a "face".

Take your sun-earth-moon model. If you mark one side of the moon and move it around the Earth with your hand, you will notice that the mark does not always face the Earth.

What do you have to do so that the mark (the "moon face") always faces the earth?

Exactly, you have to turn the moon during its orbit! And in such a way that it has rotated around itself once after exactly one orbit around the Earth. Then it always turns the same side towards the earth model.

And how long does a moon day last? That's right, 28 days, i.e. four weeks.